Add feature flag enabled by default

Add FF condition to group sidebar template
Add documentation

app/helpers/groups_helper.rb

@@ -80,6 +80,21 @@ module GroupsHelper
       .count
   end
 
+  def group_open_merge_requests_count(group)
+    if Feature.enabled?(:cached_sidebar_merge_requests_count, group, default_enabled: :yaml)
+      cached_issuables_count(@group, type: :merge_requests)
+    else
+      number_with_delimiter(group_merge_requests_count(state: 'opened'))
+    end
+  end
+
+  def group_merge_requests_count(state:)
+    MergeRequestsFinder
+      .new(current_user, group_id: @group.id, state: state, non_archived: true, include_subgroups: true)
+      .execute
+      .count
+  end
+
   def cached_issuables_count(group, type: nil)
     count_service = issuables_count_service_class(type)
     return unless count_service.present?
@@ -88,26 +103,6 @@ module GroupsHelper
     format_issuables_count(count_service, issuables_count)
   end
 
-  def issuables_count_service_class(type)
-    if type == :issues
-      Groups::OpenIssuesCountService
-    elsif type == :merge_requests
-      Groups::MergeRequestsCountService
-    end
-  end
-
-  def format_issuables_count(count_service, count)
-    if count > count_service::CACHED_COUNT_THRESHOLD
-      ActiveSupport::NumberHelper
-        .number_to_human(
-          count,
-          units: { thousand: 'k', million: 'm' }, precision: 1, significant: false, format: '%n%u'
-        )
-    else
-      number_with_delimiter(count)
-    end
-  end
-
   def group_dependency_proxy_url(group)
     # The namespace path can include uppercase letters, which
     # Docker doesn't allow. The proxy expects it to be downcased.
@@ -312,6 +307,26 @@ module GroupsHelper
   def ancestor_locked_and_has_been_overridden(group)
     s_("GroupSettings|This setting is applied on %{ancestor_group} and has been overridden on this subgroup.").html_safe % { ancestor_group: ancestor_group(group) }
   end
+
+  def issuables_count_service_class(type)
+    if type == :issues
+      Groups::OpenIssuesCountService
+    elsif type == :merge_requests
+      Groups::MergeRequestsCountService
+    end
+  end
+
+  def format_issuables_count(count_service, count)
+    if count > count_service::CACHED_COUNT_THRESHOLD
+      ActiveSupport::NumberHelper
+        .number_to_human(
+          count,
+          units: { thousand: 'k', million: 'm' }, precision: 1, significant: false, format: '%n%u'
+        )
+    else
+      number_with_delimiter(count)
+    end
+  end
 end
 
 GroupsHelper.prepend_if_ee('EE::GroupsHelper')

app/views/layouts/nav/sidebar/_group.html.haml

 - issues_count = group_open_issues_count(@group)
-- merge_requests_count = cached_issuables_count(@group, type: :merge_requests)
+- merge_requests_count = group_open_merge_requests_count(@group)
 - aside_title = @group.subgroup? ? _('Subgroup navigation') : _('Group navigation')
 - overview_title = @group.subgroup? ? _('Subgroup overview') : _('Group overview')
 

config/feature_flags/development/cached_sidebar_merge_requests_count.yml

+---
+name: cached_sidebar_merge_requests_count
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/55971
+rollout_issue_url:
+milestone: '13.11'
+type: development
+group: group::product planning
+default_enabled: true

doc/user/project/merge_requests/reviewing_and_managing_merge_requests.md

@@ -28,6 +28,39 @@ You can [search and filter the results](../../search/index.md#filtering-issue-an
 
 ![Group Issues list view](img/group_merge_requests_list_view.png)
 
+## Cached merge request count **(FREE SELF)**
+
+> - [Introduced]([link-to-issue](https://gitlab.com/gitlab-org/gitlab/-/issues/299542)) in GitLab 13.11.
+> - It's [deployed behind a feature flag](../../feature_flags.md), enabled by default.
+> - It's enabled on GitLab.com.
+> - It's recommended for production use.
+> - For GitLab self-managed instances, GitLab administrators can opt to disable it.
+
+WARNING:
+This feature might not be available to you. Check the **version history** note above for details.
+
+In a group, the sidebar displays the total count of open merge requests and this value is cached if higher
+than 1000. The cached value is rounded to thousands (or millions) and updated every 24 hours.
+
+### Enable or disable cached merge request count **(FREE SELF)**
+
+Cached merge request count in the left sidebar is under development but ready for production use. It is
+deployed behind a feature flag that is **enabled by default**.
+[GitLab administrators with access to the GitLab Rails console](../../../administration/feature_flags.md)
+can disable it.
+
+To disable it:
+
+```ruby
+Feature.disable(:cached_sidebar_merge_requests_count)
+```
+
+To enable it:
+
+```ruby
+Feature.enable(:cached_sidebar_merge_requests_count)
+```
+
 ## Semi-linear history merge requests
 
 A merge commit is created for every merge, but the branch is only merged if